Nothing wrong with your friend's reasons, OP but she is being a bit naive to think that four children always entertain each other, play beautifully together, etc (siblings are different to friends, cousins, etc). Plus she seems to have chosen to ignore all the things that are harder with four!
As a parent you cope better with more experience, i.e. you cope better with the second child than with the first because you know what to expect, etc and are more relaxed having done it before. That isn't the same as saying the two/ three/ four children are easier than one. It is just that the parent has learnt how to deal with the demands of parenthood better.
'Parenting 3 with help from your own parents/PILs/siblings etc, and a wide circle of your and DC friends (and possibly a SAH parent) is quite different to parenting 1 child with poss special needs, 2 fulltime stressful jobs, one of which involves working overseas periodically, living 160 miles away from practical family support. Amounts of money once bills are paid, amounts of physical and mental support, amounts of time to get things done, distances in commutes to work/school/shops etc, amount of interaction the DC's need - all of these can vary drastically and can fall on the plus or minus side of the argument - regardless of numbers of kids.
Can't disagree with this in principle, biddy but we aren't comparing one family with masses of support, easy jobs, etc with another with v demanding jobs and no support. In your case above, it is the external circumstances which are making parenting a challenge, not the one child.